The sensor comprises a matrix of active pixels, each pixel including a photosensitive element (most commonly a photodiode) and a number of transistors allowing the charges generated by light in the pixel to be collected in order to be converted to voltage. The sensor assembly is operated by a sequencing circuit in order to ensure that the pixels are reset and that the charges are integrated over a certain duration, starting from an instant in time of the start of integration, and lastly that the voltages representing the electric charges accumulated in the pixels are read out. These voltages are read out by a readout circuit placed at the base of each of the columns of pixels of the matrix. A row of pixels is read out by simultaneously addressing all of the pixels of this row using a row decoder; to achieve this, the pixels each comprise a row selection transistor which is turned on by a command from the row decoder, this being carried out simultaneously for all of the pixels of one and the same row. The row selection transistor then connects the pixel to a respective column conductor, common to all of the pixels of the same column of pixels, in order to transfer, to this column conductor, a useful signal representing the charges generated in the pixel located at the intersection of the selected row and the column in question. The transfer occurs simultaneously for all of the pixels of the row, each to its respective column conductor.
The radiological sensor is placed behind the human body part to be observed: for an intraoral dental radiological sensor, it is therefore placed inside the mouth of the patient, in proximity to the dental region to be observed. An X-ray source is placed outside the mouth of the patient, facing the sensor, and exposes the latter with a brief flash of X-rays through the biological tissues or other matter to be observed.
Among the significant constraints on the use of such a system, the risk of exposure of the patient and those around him or her to X-rays should be considered in particular. It is necessary to minimize the delivered dose of X-rays while still obtaining a good image of the observed region. It is for this reason that the X-ray source emits a brief flash corresponding to a limited dose of radiation.
This requires that the sensor is ready to record an image as soon as the flash is emitted, otherwise a portion of the delivered dose is wasted. However, it is also important that imaging does not start before the start of illumination, since even in the absence of X-rays the pixels collect electric charges due to the presence of dark currents in the photodiodes, i.e. currents produced even in the absence of light and hence in the absence of X-rays. These charges must be removed before the start of imaging.
An attempt is therefore made to synchronize the start of integration of useful charges in the photodiodes with the start of the X-ray flash. Similarly, an attempt is made to synchronize the end of integration of useful charges with the end of the X-ray flash.
In the prior art, multiple solutions have been used in order to achieve this synchronization.
One solution consists in using a wired connection between the sensor and the X-ray source in order to trigger the integration of an electronic image at the same time as the X-ray source is started up. It is however preferable to avoid a wired connection in the crowded medical environment in which the radiological image is taken. Furthermore, the wired connection requires a common protocol between the sensor and source, which is difficult to reconcile with the requirement for the sensor to be able to be exposed by any source, or, conversely, for the source to be able to illuminate any sensor.
It is therefore also proposed to place an X-ray detector beside the image sensor, inside the mouth or outside the mouth; this requires an additional component and a link between this component and the sensor.
In one particular situation that exclusively applies to a sensor using CCD technology and not to a sensor using MOS technology, the sensor having a CCD central charge transfer register to which the charges produced by the two halves of a sensor are transferred before being shifted stepwise towards a charge-to-voltage conversion circuit outside the matrix, it has previously been proposed (patent U.S. Pat. No. 5,510,623) not to mask the central register from light, whereas in fact it should be. The register is based on silicon and is therefore naturally photosensitive. It collects charges if it receives light and it transfers its charges stepwise to the charge-to-voltage conversion circuit. The resulting voltage level is continuously monitored; it represents dark current noise before the start of an X-ray flash; if this level increases substantially, it means that an X-ray flash has started and a complete image capture operation may be triggered. This solution is not transferable to CMOS sensors which do not have a readout charge transfer register; moreover, it interferes with the operation of the sensor by making the central register sensitive to light while it reads out the charges produced in the matrix, which negatively affects the image.
In yet another solution, using CCD technology, three X-ray detection diodes are placed behind the matrix of pixels. The resulting technology requires more manufacturing steps.
In another solution, pixels distributed throughout the matrix are used as reference pixels and are monitored in order to trigger the capture of an image if the level of a certain number of these reference pixels exceeds a threshold. This requires specific addressing means for reading out the reference pixels. This is also the case if reference zones of multiple pixels are used to carry out this detection.
In another solution, a detection cell which is larger than a pixel and capable of surrounding the entire matrix is provided for the purpose of detecting the arrival of an X-ray flash. This solution takes up space and detection potentially occurs where few X-rays arrive due to the obstacles through which they must pass.
In one particular solution, the overall image read out by the pixels is compared with an image taken in the dark before exposure to X-rays. When the read-out image suddenly becomes substantially different from the image taken in the dark, it is concluded that the flash has started. This requires that the entire matrix be read out in order to acquire this information on a sudden change in the luminosity level of the overall image.
Patent publication US 2007/0176109 recalls these various solutions, which are taken from various patent publications, and it proposes another solution using pixels for detecting the arrival of X-rays which have a quicker response time than the ordinary pixels of the matrix. These pixels are located on the periphery of the matrix and can be addressed by the same addressing means as the pixels of the matrix. They are preferably larger, and hence take up more space, than the pixels of the matrix.
In publication WO2011/008421, the matrix of pixels is read out using sub-sampling, i.e. not all of the pixels are read out; only those rows of pixels located on the periphery are actually read out in order to detect the arrival of X-rays. This complicates the internal arrangement of the sensor and its sequencing circuits.
In patent EP0757474, it is specified that the detection threshold is progressive and depends on the preceding image, in order to account for the fact that the dark current of the pixels which detect the arrival of the X-ray flash depends on the ambient temperature conditions, which can vary substantially.
In order to avoid the drawbacks of the devices of the prior art and at least to strike a better compromise between the constraints imposed by each of these devices, the invention proposes modifying the detection means present on the sensor.
An intraoral radiological image sensor using MOS technology is proposed, constructed in the following manner: it comprises a matrix of rows and columns of photosensitive pixels each comprising a photodiode and a circuit with transistors allowing the charges generated by light in the pixel to be collected and converted to voltage, with, for each column of pixels, a column conductor common to all of the pixels of the column, the column conductor being connected to a respective readout circuit for the column, and with a row-addressing circuit for addressing the pixels of a selected row and transferring, to the column conductors, useful signals arising from the pixels of the selected row and representing the illumination of these pixels.
The sensor according to the invention is characterized in that it includes, in the middle of the matrix and in place of a central column or a central row of pixels, a series of photodiodes which are all electrically connected in parallel on one side to a reference potential and on the other side to one and the same detection conductor extending along the series of photodiodes, this detection conductor being connected to a detection circuit delivering a signal for triggering the capture of an image when the detected current or the variation in this current exceeds a threshold showing that an X-ray flash has been initiated.
If the sensor is generally rectangular in shape (optionally with cut-off corners), therefore having a length and a width where the length is greater than the width, provision is made for the series of photodiodes to be positioned in place of a column or row oriented in the lengthwise direction. In most cases it is the columns (in the direction of signal collection) that are oriented in the lengthwise direction, but this is not obligatory; the series of photodiodes used to detect an X-ray flash and the detection conductor then extend in the direction of the column conductors which collect the useful signals.
The photodiodes are preferably distributed with the same spacing as the pixels in the columns or rows of pixels that surround it. These photodiodes are preferably technologically identical to the photodiodes of the pixels and they preferably have the same dimensions.
In particular embodiments, provision is also made for one or more other series of photodiodes, in columns, in rows or both, each occupying all or part of the column or row of pixels that they replace.